size (200, 200);
noStroke ();

for (int i=0; i<52; i+=4) {
  fill (5*i, 5*i, 5*i);
quad(0+i, 0+2.5*i, 0+2.5*i, 200-i, 200-i, 200-2.5*i, 200-2.5*i, 0+i);
}